PROJECT SUMMARY/ABSTRACT Infertility is a significant public health problem. According to the 2006-2010 National Survey of Family Growth, approximately 10% of US couples aged 15-44 failed to conceive after one year of trying. Unfortunately, the genetic underpinnings for the majority of the infertile cases remain unknown, hindering proper counselling and treatment efforts. As a mutational mechanism, the role for retrotransposons in infertility has been increasingly recognized. Retrotransposons, including long interspersed element type 1 (LINE1 or L1), are abundant genomic DNA sequences that amplify themselves in the genome through an RNA intermediate. To maintain the integrity of the germline genome, germ cells have evolved an elaborate regulatory network to transcriptionally and post-transcriptionally silence these unrestful genomic parasites. Perhaps more than a coincidence, all genes that have been implicated in retrotransposon control in mouse germ cells are also essential for male fertility. The general observation is, when each of these genes is mutated, mouse spermatogenesis is blocked at discrete stages and accompanied by L1 upregulation at both RNA and protein levels. A critical gap in our knowledge is whether L1 derepression is a cause or consequence of the spermatogenic arrest. How might uncontrolled and untimely activation of retrotransposons disrupt spermatogenesis? The most straightforward hypothesis is that massive L1 retrotransposition instigates genomic instability and ultimately leads to germ cell demise. So far, due to technical difficulties, few studies have attempted to test this hypothesis. Using a novel transgenic mouse model for L1 retrotransposition, we recently demonstrated, for the first time, that the derepression of retrotransposon expression does lead to increased retrotransposition in mutant spermatocytes of Mov10l1 knockout mice. In this proposal, we will take advantage of this new reporter mouse line and, in conjunction with additional infertility mouse models, seek to determine whether other stages of germ cells support L1 retrotransposition. The proposed project will provide much needed insights into L1 retrotransposition potential in discrete stages of male germ cell development and lay the foundation for a critical evaluation of the role of retrotransposition in male infertility.